How is the diagnosis of echogenic bowel made and how common is this finding on second-trimester ultrasound?
Q. A 33-year-old G2 P1 patient at 18 weeks' gestation presents to the ultrasound unit for a routine anatomic survey. Her medical and family histories are unremarkable and her pregnancy has been uncomplicated. A finding of isolated fetal echogenic bowel is noted. Fetal growth is appropriate for gestational age and no other anomalies are detected. How is the diagnosis of echogenic bowel made and how common is this finding on second-trimester ultrasound?
What is the differential diagnosis of fetal echogenic bowel observed on second-trimester ultrasound?
In fetuses with a nonpathological cause for echogenic bowel, the primary mechanism is thought to be accumulation with meconium.1 Prior studies also have demonstrated the development of echogenic bowel following invasive procedures such as amniocentesis and intrauterine fetal transfusions secondary to fetal swallowing of blood from the amniotic cavity. It has been demonstrated that this finding may even persist for 2 to 4 weeks following the procedure.4,5 Primary gastrointestinal pathology such as bowel obstruction, atresia, and perforation also may cause an echogenic appearance of the fetal bowel. In cases of obstruction and atresia, decreased meconium fluid content is the proposed cause for the increase in echogenicity; however, the presence of meconium outside the intestinal lumen likely is responsible for the echogenic appearance in cases of bowel perforation.1,3 Echogenic bowel also has been reported in cases of Hirschsprung disease.6
The estimated incidence of aneuploidy in fetuses with isolated echogenic bowel ranges from 3.3% to 16%, with trisomy 21 being the most commonly diagnosed aneuploidy in this population.3,7-11 Other karyotypic abnormalities, such as trisomy 18, trisomy 13, Turner syndrome, and chromosomal mosaicism, also have been reported.3,7 Hypoperistalsis due to mechanical or functional bowel obstruction with subsequent dehydration of meconium is the proposed mechanism causing this finding in fetuses with abnormal karyotype.12 Although the incidence of chromosomal abnormalities is higher in fetuses with additional ultrasound markers of aneuploidy, echogenic bowel as an isolated finding still confers a increased risk (likelihood ratio, 6.7) for trisomy 21 above the maternal age-adjusted baseline risk.13
Cystic fibrosis also should be considered when echogenic bowel is diagnosed. It is thought that abnormal pancreatic enzyme secretion leads to thickened meconium, and subsequent meconium ileus is observed in 10% to 20% of newborns with cystic fibrosis.14 The estimated risk association between echogenic bowel and cystic fibrosis ranges from 0% to 13% in the literature.3,4,7,9,11,15 The finding of dilated loops of bowel in addition to echogenic bowel may increase this risk to as high as 17%.16
Finally, congenital infection also has been associated with isolated echogenic bowel. Direct damage to the fetal intestinal wall with subsequent paralytic ileus/perforation versus increased free peritoneal fluid secondary to hydrops may lead to the echogenic appearance of the bowel in cases of congenital infection. Cytomegalovirus (CMV) is the most commonly observed infection, but rubella, varicella, herpes, toxoplasmosis, and parvovirus also have been reported.1,3,10,11,17 Although the majority of studies quote a 3% to 4% incidence of congenital infection in fetuses with echogenic bowel, rates as high as 10% have been reported.9,16,17 In a series of 650 cases with primary CMV infection, 7 cases (1.2%) had isolated echogenic bowel as the sole ultrasound finding.18